The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity and user bit rate for wireless systems by employing uncorrelated propagation paths for data streams, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity.
MIMO employs a number of separate independent signal paths for data streams, for example by means of several transmitting and receiving antennas. The more signal paths that are available, the more parallel data streams may be transmitted.
One problem with MIMO transmission is channel correlation and antenna coupling. These factors tend to limit the number of information streams that are possible to transmit or receive.
In the document “Reconfigurable antenna solution for MIMO-OFDM systems” by Daniele Piazza and Kapil R. Dandekar, a MIMO antenna system comprising re-configurable antennas is described. The re-configurable antennas are described as dipoles, which electrical lengths are altered by means of PIN diode switches. Thus the mutual coupling and far field radiation pattern are altered. These variable electrical characteristics allow multipath signal components to be weighted differently, and by setting the switches in a certain configuration and thus setting certain electrical characteristics, the MIMO link capacity may be enhanced.
A disadvantage of the configuration disclosed in said document is that the momentary optimal choice is determined by switching through all possible switch configurations available, and measuring the channel for each switch configuration. Each time the channel capacity decreases below a threshold, the procedure is repeated. This is a lengthy procedure that requires a lot of capacity, and which procedure, in fact, is quite impractical.
In MIMO transmission, the channel matrix has eigenvalues in the same order of magnitude. Hence in case the channel matrix has big eigenvalue spread, some of the information streams will be subject to high error rates and should therefore not be recovered. In a similar way as the channel matrix, the coupling between antenna elements defies MIMO transmission. The coupling and channel matrix can be seen as two independent matrixes. Thus, the product can be more MIMO destructive but also having an equalization effect which enhances MIMO transmission. Hence, observing the channel matrix given a certain antenna constellation results in a measure for how the antenna coupling should be adjusted. Assume that the MIMO antenna supports a means for adjusting the coupling then it can be set to produce a matrix product which is pro MIMO. The adjustment can be adaptive or by lookup.